PROJECT SUMMARY Salmonella enterica is a highly diverse species of Gram-negative bacteria that can be grouped into typhoidal and non-typhoidal serovars. Non-typhoidal serovars, such as S. Typhimurium, cause gastroenteritis and inflammatory diarrhea, whereas typhoidal serovars, such as S. Typhi, cause typhoid fever, a systemic disease with a comparatively decreased inflammatory response. However, the virulence strategies that set typhoidal Salmonella serovars apart from non-typhoidal Salmonella serovars remain understudied. Experiments proposed in this application are aimed at addressing this important gap in knowledge. Our long- range goal is to elucidate the molecular mechanisms by which typhoidal Salmonella serotypes manipulate host responses during infection. Previously, comparative analysis of Salmonella genomes revealed that typhoidal serovars contain a higher number of pseudogenes than non- typhoidal serovars, suggesting that pseudogene accumulation may contribute to the differences in disease manifestation caused by these serovars. One such pseudogene in S. Typhi is eptB, which encodes a phosphoethanolamine transferase that specifically modifies the outer keto- deoxyoctulosonate (KDO) residue of lipopolysaccharide (LPS). Our central hypothesis is that eptB pseudogene formation in S. Typhi represents a novel virulence mechanism that contributes to the stealth and immune evasion properties that characterize typhoidal Salmonella serovars. The objectives of this application are to identify the mechanism by which eptB pseudogene formation moderates the host immune response and to determine how acquisition of the eptB pseudogene synergizes with accumulation of additional pseudogenes to promote the virulence of typhoidal Salmonella serovars. To test our hypothesis and accomplish these objectives, we will examine how eptB pseudogene formation impacts binding of the host protein, intelectin, to LPS to regulate host responses to LPS (specific aim 1) and also determine how eptB pseudogene formation offsets the acquisition of other typhoidal pseudogenes to enhance Peyer's patch colonization (specific aim 2). Our analysis of this novel virulence mechanism in typhoidal Salmonella serovars will be useful and necessary to understand how the interplay between pathogen and the innate immune system gives rise to responses that distinguish typhoid fever from gastroenteritis, thereby ushering in a significant conceptual advance.